Structure and bonding in actinide complexes
Actinides and their binary compounds are of tremendous technological importance. They find application as fuels for nuclear power production, power supplies for deep-space exploration, and in nuclear weapons. The storage and disposal of used nuclear fuels, the disposition of facilities and wastes from nuclear weapons production sites, the transport of actinides at environmental contamination sites, and the storage lifetime of nuclear weapons pose a variety of problems that require research so that we may properly handle the material or site of concern.
The deceptively simple binary formula of the tetravalent actinide oxides, AnO2, hides an incredibly complex structural nature, and a tendency to form nonstoichiometric phases (AnO2±x). For plutonium (Pu), it was a widely held view that compositions with oxygen/plutonium ratios higher than 2.0 were not stable. This view was challenged when the reaction of plutonium metal or oxide with water vapor was shown to produce PuO2+x. This new material was examined by X-ray absorption fine structure (XAFS) spectroscopy and found to be comprised of a mixed-valent solid of Pu(IV)/Pu(V) oxidation states with short “plutonyl-like” Pu=O bonds of 1.85 angstroms (Å).
We are currently investigating similar behavior
in mixed-valent uranium oxides of general formula UO2+x. UO2 and U4O9
(UO2.25) have been synthesized and studied by XAFS, along with several
intermediate compositions of UO2+x. We have found that as x increases,
a new feature at 1.74 Å,
which has been assigned as U=O, also increases. An example of this new feature
These short contacts are not consistent with what has been previously reported using neutron diffraction and pair distribution function; however, these techniques look for long-range order, and this new short contact is a small percentage and probably highly disordered so it would not be observed by those techniques. It is only by using XAFS, which looks at local environments, that we have been able to observe the formation of this new type of bonding in a uranium oxide material. We have established XAFS capabilities at the Stanford Synchrotron Radiation Laboratory beamline 11-2 for uranium and transuranic samples.
We are also investigating the extent of covalent bonding in actinide complexes in a variety of compounds, including the isostructural UCl6x- series (x = 1, 2, 3). Our results challenge the dogma of actinide chemistry that 5f electrons don’t contribute to the formation of covalent bonds. In fact, our data show that UCl62- has about half the covalency found in transition metals with contributions from both the 6d and 5f orbitals. We plan to extend out analysis beyond chlorine (Cl) edges to include oxygen (O) and nitrogen (N) edges, which would require ultra-high vacuum and a special chamber. Improving our understanding of the extent of covalent bonding can aid in ligand design for fuel separation and reprocessing.